Nematic liquid-crystal composition

ABSTRACT

The invention relates to nematic liquid-crystal compositions comprising bisalkenyl compounds of the formula I  
                 
 
     and to their use in plasma-addressed LCDs.

BACKGROUND OF THE INVENTION

[0001] The invention relates to nematic liquid-crystal compositionsbased on bisalkenyl compounds and terminally fluorinated compounds.These compositions are particulary suitable for plasma-addressed displaydevices.

[0002] Plasma-addressed displays (PADs) are of great utility forhigh-information displays, which are of commercial interest. Such PADsare used in TV applications and in high-information displays forcomputer screens, automobiles and aircraft.

[0003] PADs have electrical switching elements and a plasma cell with aplurality of addressing channels which is connected to the display cell.

[0004] Such PADs are disclosed, for example, in WO 96/00925, EP 0 628944, EP 0 545 569, U.S. Pat. No. 4,896,149 and U.S. Pat. No. 5,077,553.

[0005] In a PAD, the switching elements are addressed via a multiplexingscheme. This charges the electrodes of a pixel in the limited timeduring which they are active. They subsequently become and remaininactive until they are addressed again in the next cycle. Consequently,the voltage change at a plasma-addressed pixel is an unwanted, but verycrucial characteristic of such a display. Discharge of the electrodes ofthe pixel is determined by two factors, the capacity of the pixel andthe specific resistance of the liquid-crystal material, i.e., the liquidcrystal, between the electrodes.

[0006] Conventional liquid-crystalline materials for active matrixdisplays (AMDs) are unsuitable for PADs since they have excessively highvalues for the dielectric anisotropy.

[0007] There therefore continues to be a great demand for liquid-crystalcompositions having high specific resistance and having other materialproperties which are suitable for use in PADs, such as, for example, abroad nematic mesophase range with an extremely low smectic-nematictransition temperature and the absence of crystallization at lowtemperature.

[0008] A further requirement in such compositions is for high steepnessof the characteristic lines (small difference between V₉₀ and V₁₀).

SUMMARY OF THE INVENTION

[0009] An object of the invention is to provide liquid-crystalcompositions which have very high specific resistance and which alsosatisfy the other requirements.

[0010] An additional requirement is for compositions having low Δnvalues of <0.1 and rotational viscosities of ≦150 mpa·s at 20° C.,preferably ≦120 mPa·s, in particular ≦100 mpa·s.

[0011] It has now been found that nematic liquid-crystal compositionsfor plasma addressed liquid crystal displays (PALCDS) comprising atleast one compound of the formula I

[0012] in which

[0013] R¹ and R², independently of one another, are alkenyl oralkenyloxy having 1 to 8 carbon atoms,

[0014] each Z¹ and Z², independently of one another, and if Z¹ ispresent a number of times, these radicals also independently of oneanother, are a single bond, —(CH₂)₂— —COO— , —OCO—, trans-CH═CH—, —CH₂O—or —OCH₂—, where Z² is preferably a single bond,

[0015] each A¹, A² and A³, independently of one another, and if A¹ ispresent a number of times, these radicals also independently of oneanother, are trans-1,4-cyclohexylene, 1,4-cyclohexenylene, in which alsoone or two nonadjacent CH₂ groups may be replaced by oxygen,1,4-phenylene, 2-fluoro-1,4-phenylene, 3-fluoro-1,4-phenylene,2,3-difluoro-1,4-phenylene, 3,5-difluoro-1,4-phenylene,2,6-difluoro-1,4-phenylene or 1,5-pyridyl or 1,5-pyrimidyl, in which oneor two H atoms are optionally replaced by F,

[0016]  where at least one of the rings A² is preferably atrans-1,4-cyclohexylene rings, and particularly preferably both rings A²and A³ are trans-1,4-cyclohexylene,

[0017] and

[0018] n is 0, 1 or 2,

[0019] are particularly suitable for PAD applications. Z² is preferablya single bond, and at least one of the rings A² and A³ is preferably atrans-1,4-cyclohexylene ring. Particularly preferably, both rings A² andA³ are trans-1,4-cyclohexylene. In PADs, very high values for the RCtime (i.e., the time constant for the discharge of the pixel electrodes)can be achieved. These compositions likewise have reduced viscosity andenable AMDs to be operated at the first transmission minium, and exhibitno crystallization and no occurrence of smectic phases when stored intest cells of appropriate cell gap for times of at least 100 hourspreferably 500 hours and most preferably 1000 hours at a temperature of−20° C., preferably at −30° C. and most preferably of −40° C.

[0020] Preference is furthermore given to liquid-crystal mixtures whichcomprise at least one compound of the formula II

[0021] in which

[0022] R³ is alkyl, alkenyl, alkyloxy or alkenyloxy having 1-8 carbonatoms,

[0023] Z³ and Z⁴, independently of one another, are a single bond,(CH₂)₂, OCH₂, CH₂O, COO, CN, F, CF₂H or CF₃

[0024] X is OCF₂H, OCF₃, OCH₂CF₃, OCHFCF₃, OCF₂CF₂H or F

[0025] L is H or F

[0026] and

[0027] A⁴ is trans-1,4-cyclohexylene, 1,4-phenylene,3-fluoro-1,4-phenylene or 3,5-difluoro-1,4-phenylene.

[0028] Particular preference is given to nematic liquid-crystalcompositions which comprise at least one compound of the formula Ia

[0029] in which

[0030] R¹ and R² ₁ independently of one another, are alkenyl oralkenyloxy having 1 to 8 carbon atoms,

[0031] and

[0032] Z² is a single bond, —(CH₂)₂— or trans-CH═CH—.

[0033] Z² is particularly preferably a single bond.

[0034] Nematic liquid-crystal compositions of the present inventionfurthermore preferably comprise at least one compound of the formula III

[0035] in which

[0036] R⁴ and R⁵, independently of one another,

[0037]  are alkyl, alkoxy, alkenyl or alkenyloxy having 1 to 10 carbonatoms, in which 1 or a maximum of 2 non-adjacent CH₂ groups may bereplaced by O,

[0038] each Z⁵ if it is present a number of times independently of oneanother, is a single bond, COO, OCO, (CH₂) ₂, OCH₂, CH₂O or trans-CH═CH,

[0039] each A⁵ and A⁶ independently of one another, and if A⁵ is presenta number of times these radicals also independently of one another,

[0040]  are trans-1,4-cyclohexylene, 1,4-cyclohexenylene, in which alsoone or two nonadjacent CH₂ groups may be replaced by oxygen,1,4-phenylene, 2-fluoro-1,4-phenylene, 3-fluoro-1,4-phenylene,2,3-difluoro-1,4-phenylene, 3,5-difluoro-1,4-phenylene,2,6-difluoro-1,4-phenylene or 1,5-pyrimidinedyl, in which one or two Hatoms are optionally replaced by F,

[0041] and

[0042] n is 1, 2 or 3.

[0043] Very particular preference is given to nematic liquid-crystalcompositions which comprise at least one compound of the formula Iaa

[0044] in which

[0045] R¹ and R², are each, independently of one another, CH═CH₂,CH═CH—CH₂—CH₃ and/or CH═CH—CH₃

[0046] and

[0047] Z² is (CH₂)₂ or a single bond.

[0048] Z² is preferably a single bond and R¹ and R² are preferablyidentical to each other.

[0049] Preference is furthermore given to nematic liquid-crystalcompositions which, besides compounds of the formula Iaa, comprise atleast one further compound of the formula I

[0050] and/or of the formula II

[0051] In a preferred embodiment, the compositions according to theinvention comprise from 10 to 34% of two or more compounds of theformula I.

[0052] The nematic compositions particularly preferably comprise atleast two compounds of the formula Iaa from the group consisting of theformulae Ia1 to Ia9:

[0053] The symmetrically substituted compounds of the formula Iaa, i.e.,the compounds Ia1, Ia3, Ia5 and Ia8, are preferred, and of these, veryparticular preference is given to the 1-alkenyl compounds Ia1, Ia3 andIa5. The best response times are achieved when using Ia1, Ia2 and Ia3.The most preferred compounds are those of the formulae Ia1 and Ia5.

[0054] The LC compositions according to the invention have a dielectricanisotropy, measured at 20° C. and 1 kHz, of less than 4.0, preferablybetween 1.0 and 3.0, in particular from 1.5 to 2.5. The LC compositionsaccording to the instant invention have a birefringence at 20° C. and589 nm of not more than 0.1, preferably not more than 0.08, especiallypreferred of not more than 0.075 and most preferred of below 0.07. Thevoltage holding ratio HR after 5 minutes at 100° C. and IV using acommercial equipment of Antronic-Melchers, Germany, is at least 98%,preferably higher than 98.5%, especially preferred higher than 99.0% andmost preferred at least 99.5%. The clearing point of the compositionsaccording to the invention is above 75° C., in particular above 80° C.,very particularly above 90° C.

[0055] Compositions of this type preferably comprise two, three or fourcompounds of the formula I. Preferred compositions comprise at least ofmore than 10% by weight of the individual compounds of the formula I.

[0056] The compounds of the formula I are disclosed in the German patentapplications and can be prepared analogously to known compounds.

[0057] The compounds of the formulae II and III are known to the personskilled in the art.

[0058] Preferred compositions likewise comprise two or more compounds ofthe formula III. Preference is given here to the compounds selected fromthe group consisting of the formulae IIIa and IIIb:

[0059] in which R⁴ and R⁵ are as defined above under the formula III.

[0060] In a preferred embodiment, the compositions according to theinvention comprise at least two compounds of the formula IIIa.

[0061] The compositions according to the invention preferably comprisefrom 6 to 48% by weight, in particular from 10 to 34% by weight or from16 to 30% by weight, of components of the formula I. The percentages byweight for the other groups are preferably in the following ranges:

[0062] Formula II: from 10 to 45%, in particular from 18 to 38% FormulaIII: from 5 to 52%, in particular from 12 to 40%.

[0063] The components of the formulae I, II and II preferably form thebasis of the claimed compositions and together make up at least 60% byweight, particularly preferably at least 75% by weight, of thecompositions However, it is also possible to use further LC componentsin smaller percentages in addition to the components of the formulae Ito III for fine adjustment of the claimed compositions. In this case,compounds having a high clearing point are used in particular.

[0064] In general, the compositions consist of a plurality of compoundsmixed in a conventional manner. In general, the desired amount of thecomponents used in the smaller amount are dissolved in the componentsmaking up the principal constituent, advantageously at elevatedtemperature. If the temperature selected is above the clearing point ofthe principal constituent, completion of the dissolution process isparticularly easy to observe.

[0065] However, it is also possible to mix the components in a suitablesolvent, for example in acetone, chloroform or methanol, and, aftermixing, to remove the solvent again, for example by distillation underreduced pressure. It goes without saying that it must be ensured herethat the solvent does not introduce any impurities or undesired dopingsubstances.

[0066] By means of suitable additives, the liquid-crystal phasesaccording to the invention can be modified in such a way that they canbe used in any type of PAD which has been disclosed hitherto.

[0067] Without further elaboration, it is believed that one skilled inthe art can, using the preceding description, utilize the presentinvention to its fullest extent. The following preferred specificembodiments are, therefore, to be construed as merely illustrative, andnot limitative of the remainder of the disclosure in any way whatsoever.

[0068] In the foregoing and in the following examples, all temperaturesare set forth uncorrected in degrees Celsius and unless otherwiseindicated, all parts and percentages are by weight.

[0069] The entire disclosure of all applications, patents andpublications, cited above and below, and of corresponding Germanapplication No. 197 33 199.8, filed Aug. 1, 1997 is hereby incorporatedby reference.

[0070] In order to characterize the liquid-crystalline compounds used inthe compositions according to the invention. The following abbreviationsare used in the examples. The indices n and m denote the number ofcarbon atoms present in the terminal alkyl radicals.

[0071] The physical properties are the values at 20° C., unlessexplicitly stated otherwise.

[0072] In the present application and in the examples below, thestructures of the liquid-crystal compounds are indicated by means ofacronyms, the transformation into chemical formulae taking place inaccordance with Table A and Table B below. All radicals C_(n)H_(2n+1)and C_(m)H_(2m+1) are straight-chain alkyl radicals having n or m carbonatoms respectively. The coding in Table B is self-evident. In Table A,only the acronym for the parent structure is given. In individual cases,the acronym for the parent structure is followed, separated by a dash,by a code for the substituents R¹, R², L¹ and L²: Code for R¹, R², L¹,L² R¹ R² L¹ L² nm C_(n)H_(2n+1) C_(m)H_(2m+1) H H nOm C_(n)H_(2n+1)OC_(m)H_(2m+1) H H nO.m OC_(n)H_(2n+1) C_(m)H_(2m+1) H H n C_(n)H_(2n+1)CN H H nN.F C_(n)H_(2n+1) CN H F nF C_(n)H_(2n+1) F H H nOFOC_(n)H_(2n+1) F H H nCl C_(n)H_(2n+1) Cl H H nF.F C_(n)H_(2n+1) F H FnmF C_(n)H_(2n+1) C_(m)H_(2m+1) F H nCF₃ C_(n)H_(2n+1) CF₃ H H nOCF₃C_(n)H_(2n+1) OCF₃ H H nOCF₂ C_(n)H_(2n+1) OCHF₂ H H nS C_(n)H_(2n+1)NCS H H rVsN C_(r)H_(2r+1)—CH═CH—C_(s)H_(2s)— CN H H rEsNC_(r)H_(2r+1)—C_(s)H_(2s)— CN H H nAm C_(n)H_(2n+1) COOC_(m)H_(2m+1) H HnF.Cl C_(n)H_(2n+1) Cl H F

[0073] TABLE A

[0074] TABLE B

[0075] An denotes the optical anisotropy (589 nm, 20° C.), Ae denotesthe dielectric anisotropy (1 kHz, 20° C.), H.R. denotes the voltageholding ratio (at 100° C., after 5 minutes in the oven, 1 V), V_(0,0,20)denotes the threshold voltage determined at 20° C.

EXAMPLE 1

[0076] A mixture of the following composition was prepared andinvestigated for usefulness in PALCDs. Abbreviation % by weight CC-V-V14.0 CC-2V-V2 10.0 ECCP-3F.F 5.0 ECCP-5F.F 5.0 CCP-2F.F.F 6.0 CCP-3F.F.F6.0 CCP-5F.F.F 6.0 CCH-303 13.0 CCH-501 13.0 CP-33 5.0 CP-43 5.0 CCPC-336.0 CCPC-34 6.0 100.0 This composition has the following properties: T(S, N) <20° C. T (N, I) 97.0° C. Rot. visc. γ₁ (20° C.) 99 mPa · s Δn(20°C., 589 nm) 0.0672 Δε (20° C., 1 kHz) 1.9 ε_(II) (20° C., 1 kHz) 5.0K₁ (20° C.) 13.2 10⁻¹² N K₃/K₁ (20° C.) 1.04 V_(0,0,20) 2.82 V HR (100°C., 5 min, 1 V) 99.6%

[0077] The threshold voltage V_(0,0,20) was determined at 20° C. (fromdielectric measurements).

[0078] The voltage holding ratio (HR) was determined using anAutronic-Melchers measurement instrument as described in the Merck GroupLiquid Crystal Newsletter No. 9, October 1992, by T. Jacob and U.Finkenzeller.

COMPARATIVE EXAMPLE 1

[0079] The liquid-crystal mixture having a similar clearing point wasprepared analogously to Example 1, but without using any compounds ofthe formula I. Composition Abbreviation % by weight PCH-301 7.0 PCH-3023.0 ECCP-3F.F 5.0 ECCP-5F.F 5.0 CCP-2F.F.F 6.0 CCP-3F.F.F 6.0 CCP-5F.F.F6.0 CCH-301 14.0 CCH-303 13.0 CCH-501 13.0 CP-33 5.0 CP-43 5.0 CCPC-334.0 GCPC-34 4.0 CCPC-35 4.0 100.0 Properties T (S, N) <−20° C. T (N, I)93° C. Rot. visc. γ₁ (20° C.) 122 mPa · s Δn (20° C., 589 nm) 0.0668 Δε(20° C., 1 kHz) 1.9 ε_(II) (20° C., 1 kHz) 5.2 K₁ (20° C.) 14.7 · 10⁻¹²N K₃/K₁ (20° C.) 1.12 V_(0,0,20) 2.93 V HR (100° C., 1 V) 97.1%

[0080] Compared with Comparative Example 1, Example 1 according to theinvention, with a higher clearing points, is characterized by asignificantly better threshold voltage and in particular by aconsiderably better rotational viscosity. The mixture according to theinvention thus switches significantly more quickly. In addition, its HRis significantly higher, i.e. better for plasma addressing.

[0081] The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

[0082] From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:
 1. A nematic liquid-crystal composition comprisingat least one compound of the formula I

in which R¹ and R², are each independently alkenyl or alkenyloxy having1 to 8 carbon atoms, each Z¹ and Z², is independently a single bond,—(CH₂)₂— —COO—, —OCO—, trans-CH═CH—, —CH₂O— or —OCH₂—, each A¹, A² andA³, is independently trans-1,4-cyclohexylene, 1,4-cyclohexenylene, inwhich also one or two nonadjacent CH₂ groups may be replaced by oxygen,1,4-phenylene, 2-fluoro-1,4-phenylene, 3-fluoro-1,4-phenylene,2,3-difluoro-1,4-phenylene, 3,5-difluoro-1,4-phenylene,2,6-difluoro-1,4-phenylene, 1,5-pyridyl or 1,5-pyrimidyl, in which oneor two H atoms are optionally replaced by F, and n is 0, 1 or 2, whichcomposition has been optimized for PALCDs.
 2. A composition according toclaim 1, further comprising at least one compound of the formula II

in which R³ is alkyl, alkenyl, alkyloxy or alkenyloxy having 1-8 carbonatoms, Z¹ and Z², are each independently a single bond, (CH₂)₂, OCH₂,CH₂O, COO, CN, F, CF₂H or CF₃ X is OCF₂H, OCF₃, OCH₂CF₃, OCHFCF₃,OCF₂CF₂H or F L is H or F and A⁴ is trans-1,4-cyclohexylene,1,4-phenylene, 3-fluoro-1,4-phenylene or 3,5-difluoro-1,4-phenylene. 3.A composition according to claim 1, further comprising at least onecompound of the formula Ia

in which R¹ and R², are each independently alkenyl or alkenyloxy having1 to 8 carbon atoms, and Z² is a single bond, —(CH₂)₂— or trans-CH═CH—.4. A composition according to claim 1, further comprising at least onecompound of the formula III

in which R⁴ and R⁵, are each independently alkyl, alkoxy, alkenyl oralkenyloxy having 1 to 10 carbon atoms, in which 1 or a maximum of 2non-adjacent CH₂ groups may be replaced by O, each Z¹ is independently asingle bond, COO, OCO, (CH₂)₂, OCH₂, CH₂O or trans-CH═CH—, each A⁵ andA⁶ is independently trans-1,4-cyclohexylene, 1,4-cyclohexenylene, inwhich also one or two nonadjacent CH₂ groups may be replaced by oxygen,1,4-phenylene, 2-fluoro-1,4-phenylene, 3-fluoro-1,4-phenylene,2,3-difluoro-1,4-phenylene, 3,5-difluoro-1,4-phenylene,2,6-difluoro-1,4-phenylene or 1,5-pyrimidinediyl, in which one or two Hatoms are optionally replaced by F, and n is 1, 2 or
 3. 5. A compositionaccording to claim 1, comprising at least one compound of the formulaIaa

in which R¹ and R², are each independently CH═CH₂, CH═CH—CH₂—CH₃ and/orCH═CH—CH₃ and Z² is —(CH₂)₂— or a single bond.
 6. A compositionaccording to claim 3, comprising at least one compound of the formula I′


7. A composition according to claim 5, further comprising at least onecompound of the formula II


8. A plasma-addressed liquid crystalline display, containing a nematicliquid-crystal composition comprising at least one compound of theformula I

in which R¹ and R², are each independently alkenyl or alkenyloxy having1 to 8 carbon atoms, each Z¹ and Z², is independently a single bond,—(CH₂)₂— —COO—, —OCO—, trans-CH═CH—, —CH₂O— or —OCH₂—, each A¹, A² andA³, is independently trans-1,4-cyclohexylene, 1,4-cyclohexenylene, inwhich also one or two nonadjacent CH₂ groups may be replaced by oxygen,1,4-phenylene, 2-fluoro-1,4-phenylene, 3-fluoro-1,4-phenylene,2,3-difluoro-1,4-phenylene, 3,5-difluoro-1,4-phenylene,2,6-difluoro-1,4-phenylene, 1,5-pyridyl or 1,5-pyrimidyl, in which oneor two H atoms are optionally replaced by F, and n is 0, 1 or
 2. 9. Adisplay according to claim 1, wherein the nematic liquid-crystalcomposition further comprises at least one compound of the formula II

in which R³ is alkyl, alkenyl, alkyloxy or alkenyloxy having 1-8 carbonatoms, Z¹ and Z², are each independently a single bond, (CH₂)₂—, OCH₂,CH₂O, COO, CN, F, CF₂H or CF₃ x is OCF₂H, OCF₃, OCH₂CF₃, OCHFCF₃,OCF₂CF₂H or F L is H or F and A⁴ is trans-1,4-cyclohexylene,1,4-phenylene, 3-fluoro-1,4-phenylene or 3,5-difluoro-1,4-phenylene. 10.A display according to claim 1, wherein the nematic liquid-crystalcomposition further comprises at least one compound of the formula Ia

in which R¹ and R², are each independently alkenyl or alkenyloxy having1 to 8 carbon atoms, and Z² is a single bond, —(CH₂)₂— or trans-CH═CH—.11. A display according to claim 1, wherein the nematic liquid-crystalcomposition further comprises at least one compound of the formula III

in which R⁴ and R⁵, are each independently alkyl, alkoxy, alkenyl oralkenyloxy having 1 to 10 carbon atoms, in which 1 or a maximum of 2non-adjacent CH₂ groups may be replaced by O, each Z¹ is independently asingle bond, COO, OCO, (CH₂)₂, OCH₂, CH₂O or trans-CH═CH, each A⁵ and A⁶is independently trans-1,4-cyclohexylene, 1,4-cyclohexenylene, in whichalso one or two nonadjacent CH₂ groups may be replaced by oxygen,1,4-phenylene, 2-fluoro-1,4-phenylene, 3-fluoro-1,4-phenylene,2,3-difluoro-1,4-phenylene, 3,5-difluoro-1,4-phenylene,2,6-difluoro-1,4-phenylene or 1,5-pyrimidinediyl, in which one or two Hatoms are optionally replaced by F, and n is 1, 2 or
 3. 12. A displayaccording to claim 1, wherein the nematic liquid-crystal compositioncomprises at least one compound of the formula Iaa

in which R¹ and R², are each independently CH═CH₂, CH═CH—CH₂—CH₃ and/orCH═CH—CH₃ and Z² is —(CH₂)₂— or a single bond.
 13. A display accordingto claim 3, wherein the nematic liquid-crystal composition comprises atleast one compound of the formula I′


14. A display according to claim 5, wherein the nematic liquid-crystalcomposition further comprises at least one compound of the formula II